1. Field of the Invention
This invention relates to a spectrally sensitized silver halide photographic emulsion, more particularly, to a silver halide photographic emulsion suitable for preparing a red-sensitive layer of a multilayer color light-sensitive material.
2. Description of the Prior Art
Spectral sensitization is a very important procedure for extending the spectral sensitivity of a silver halide photographic emulsion to wavelengths longer than the inherent sensitivity region of the silver halide itself to yield spectral sensitivity in the green and red regions, as well as to increase the overall sensitivity (to white light). In particular, multilayer color light-sensitive materials require photographic emulsion layers respectively sensitive to blue, green and red, and spectral sensitization is, therefore, indispensable to obtain photographic emulsion layers respectively sensitive to green and red.
Moreover, for color light-sensitive materials, good color reproducibility is necessary, and also from this viewpoint, the spectral sensitivity characteristics of the green-sensitive layer and red-sensitive layer are of importance. In particular, the selection of the wavelength at which the red-sensitive layer has a sensitization maximum is of significance, since it delicately affects the reproduction of flesh tints, which is important in color photography, and also affects variations in the color balance due to light sources as is described in Japanese Patent Publication No. 6207/74. For example, it is described in Japanese Patent Publication No. 6207/74 that when about 605 nm (.+-.5 nm) is selected for the wavelength at which the red-sensitive layer of the multilayer color light-sensitive material has a sensitization maximum, color balance is best kept with respect to three types of illumination, i.e., sunlight, a tungsten lamp and a fluorescent lighting. However, this selection of the wavelength at which the red-sensitive emulsion layer has a sensitization maximum cannot provide high sensitivity, although variations in the color balance with respect to these three types of light sources is minimized. Thus, for preparing a highly sensitive color light-sensitive material for use in photography, it is necessary to increase the size of the silver halide grains in the photographic emulsion in order to increase sensitivity, whereby graininess is encountered and sharpness is sacrificed.
It was found that in order to obtain highly sensitive color light-sensitive materials having satisfactory color reproducibility without suffering the above defects, it is most advantageous that the wavelength at which the red-sensitive layer has a sensitization maximum ranges from 625 nm to 645 nm, and the spectral sensitivity in the wavelength region of 580 to 600 nm is at least 40% of the maximum spectral sensitivity. There are known many sensitizing dyes which give a sensitization maximum in the above wavelength region, but the photographic emulsions using known sensitizing dyes cannot provide a sufficiently high spectral sensitivity at wavelengths of 580 nm to 600 nm.
In addition, since present-day multilayer color light-sensitive materials contain color image-forming couplers in the photographic emulsion layers, it is required that the spectral sensitizing effect of the sensitizing dyes not be inhibited by the presence of the couplers. Another requirement is freedom from coloring due to dye remaining after photographic processing, because such coloring remarkably deteriorates the hue of the color photograph obtained.